Mass spectrometers have been used to analyze a wide range of materials, including organic substances such as pharmaceutical compounds, environmental compounds and biomolecules. They are particularly useful, for example, for DNA and protein sequencing. In such applications, there is an ever increasing desire for high mass accuracy, as well as high resolution of analysis of sample ions by the mass spectrometer, notwithstanding the short time frame of modern separation techniques such as gas chromatography/mass spectrometry (GC/MS), liquid chromatography/mass spectrometry (LC/MS) and so forth.
Ion storage type mass analyzers, such as RF quadrupole ion trap, ICR (Ion Cyclotron Resonance), orbitrap, and FTICR (Fourier Transform Ion Cyclotron Resonance) mass analyzers, function by transferring generated ions via an ion optical means to the storage/trapping cells on the mass analyzer, where the ions are then analyzed. One of the major factors that limit the mass resolution, mass accuracy and the reproducibility in such devices is space charge, which can alter the storage, trapping conditions, or ability to mass analyze the contents of an ICR or ion trap, from one experiment to the next, and consequently vary the results attained.
Space charge effects arise from the influence of the electric fields of trapped ions upon each other. The combined or bulk charge of the final population of ions causes shifts in frequency and therefore m/z (i.e., dimensionless mass-to-charge ratio). At very high levels of space charge, the obtainable resolution will deteriorate and peaks close in frequency (m/z) can at least partially coalesce. A significant scan to scan variation in the magnitude of the space charge effect arises from differences in trapped ion density, caused by changes in the number of ions within the cell from one ionization/ion injection event to the next. Unless space charge is either taken into account or regulated, high mass accuracy, precision mass and intensity measurements can not be reliably achieved.
The flux of ions available for storage or trapping or mass analysis can depend on the type of ionization source employed. Different ion sources, including discontinuous and continuous types, can be used in conjunction with mass analyzers. Discontinuous ion sources generally provide discrete ion pulses or packets separated by periods when ionization events are absent or minimized. Common examples of such sources are the matrix assisted laser desorption ionization (MALDI) ion source, or the surface enhanced desorption ionization (SELDI) ion source, both of which use high-power pulsed lasers to desorb analyte molecules from a surface. A well-known and important example of a technique that produces an essentially continuous supply of ions is the electrospray ionization technique, in which singly or multiply charged ions in the gas phase are produced from a solution at atmospheric pressure.
In contrast to the potentially wide variability in ion flux delivered by ion sources, high-precision mass analyzers often require a fairly restricted range of ion population for optimal performance. If a too-small ion population is injected into the mass analyzer, it can be difficult to differentiate the detected population of ions from the noise level. Although increasing the population of ions in the analysis chamber of the mass analyzer can avoid this problem, too great an increase can lead to space charge problems as noted above, resulting in deterioration in m/z assignment accuracy. Thus, in general, the optimum performance of the ion trap mass analyzer is achieved when the ion population is characterized by maximum signal/noise ratio, but still is below the threshold of onset of significant space charge effects.
One way to improve the reproducibility of results, the mass resolution and accuracy in ion storage type devices is to control the ion population that is stored/trapped, or otherwise confined, and subsequently analyzed in the mass analyzer. Thus, ion gating techniques are generally used so as to control the total number of ions that enter an ion trap. For any particular flux of ions from a source, the so-called injection time, or time that the gate is “open” so as to allow ions to pass therethrough to a destination, may be chosen so as to allow a suitable total number of ions—for instance, 30000 ions—to pass through the gate to the destination. The destination may be an ion trap or, in fact, any apparatus capable of receiving, storing, measuring or otherwise handling ions, such as, for instance, a mass analyzer or an ion detector, an ion lens, an ion guide, etc.). Otherwise, the gate is “closed” so as to prevent ions from proceeding through to the destination.
FIGS. 1A-1B illustrate the construction and operation of a conventional ion gate 100. The conventional ion gate 100 comprises a first electrode portion 102a of length L and a second electrode portion 102b, also of length L, separated from one another so as to define an aperture 103. Ions provided by some origin 108, such as an ionization source, are accelerated in the direction of the gate 100 as ion beam 104. The ion gate 100 may be either maintained in an “ON” state, as illustrated in FIG. 1A, or, alternatively, in an “OFF” state, as illustrated in FIG. 1B, these terms being taken to mean, as used in this disclosure, that ions are permitted or are not permitted, respectively, to pass through the ion gate 100 towards a destination 110 on the opposite side of the gate from the origin.
FIG. 1A schematically illustrates ion trajectories with the ion gate 100 maintained in an ON state (or, more simply put, ion trajectories when the ion gate 100 is “ON”). When the ion gate is ON, both the first electrode portion 102a and the second electrode portion 102b are maintained at similar constant DC voltages. For simplicity of discussion, it is assumed, in the following discussion, that both electrode portions 102a-102b are maintained at the same voltage, V0. Consequently, the ions originating from origin 108 pass through the aperture 103 in the gate as ion beam 105 and continue to move away from the opposite side of the gate towards the destination 110 as ion beam 106.
FIG. 1C schematically illustrates additional details of the passage of ion beam 105 through aperture 103 in ion gate 100. Since the ion beams 104-106 comprise ions having a range of m/z ratios and, since each ion has substantially identical kinetic energy to every other ion, relatively heavier ions travel more slowly through the aperture 103 than do relatively lighter ions of the same charge. FIG. 1C shows the trajectories of, for instance, two particular ions comprising the ion beam 105 and assumed to enter the aperture 103 at the same time, each ion having a charge of unity but one ion (i.e., the ion represented by trajectory 105a) having a mass number of 100 and the other ion (i.e., the ion represented by trajectory 105b) having a mass number of 1000. Although only two species are illustrated in FIG. 1C, the ion beam 105 will, in general, comprise many species having a range of m/z ratios. FIG. 1C shows that, in the time that the lighter ion just completely passes through the aperture 103, the heavier ion only travels approximately one-third of the distance through the aperture 103.
FIG. 1B schematically illustrates ion trajectories with the ion gate 100 maintained in an OFF state (or, more simply put, ion trajectories when the ion gate 100 is “OFF”). When the ion gate is OFF, the first electrode portion 102a is maintained at voltage V0 and the second electrode portion 102b is maintained a voltage of V0+Voff. The offset voltage Voff may be either positive or negative. Assuming that V0 is positive and that Voff is negative, then, in this configuration, the positive ions comprising beam 105, including those particular ions represented by trajectories 105a and 105b, are deflected away from the first electrode portion 102a and are drawn toward the second electrode portion 102b in a fashion such that none of the ions pass through aperture 103 and whereby ions may, in fact, be neutralized at the second electrode portion 102b. The trajectories of negative ions would be reversed, such that the negative ions would be deflected away from the second electrode portion 102b and drawn towards and neutralized at the first electrode portion 102a. In this situation, ions are prevented from reaching the destination 110.
When the conventional ion gate 100 is switched from the OFF state, as shown in FIG. 1B, to the ON state, as shown in FIG. 1A, the greater velocity of relatively lighter ions will cause these to arrive at the destination 110 in advance of relatively heavier ions. More generally, assuming that all ions are initially allowed to proceed through gate 100 in the direction of destination 110 at the same time, those ions having a lesser value of the quantity m/z will arrive at the destination 110 in advance of those ions that have a greater value of m/z. When the ion gate 100 is switched in the reverse sense, from ON to OFF, the effect of ion mass will be much weaker in determining the time that ions stop arriving at destination 110, since virtually any deflection will prevent virtually all ions from proceeding to the destination 110, regardless of mass.
As a consequence of the principles described in the foregoing, the conventional gate 100 may lead to a sampling bias, wherein ions of low values of m/z are present at the destination in excess of their original abundance at the origin 108. It has been generally observed that this phenomenon is only problematical when the injection time (i.e., the time during which the gate is ON so as to permit a pulse of ions to pass) is so short that it approaches the flight time across the width of the gate.
FIG. 2 graphically depicts how the lag of ions having high values of m/z can cause anomalous mass spectrum measurements when the conventional ion gate is operated for short gate times. In FIG. 2, plots are given of calculations of the total number of ions detected for a situation in which an ion pulse is produced by passing an ion beam having equal concentrations of two species of ions—one having an m/z value equal to 100 and the other one having an m/z value equal to 1000. Curve 205a in FIG. 2 represents the calculated total number of ions having m/z equal to 100 that are detected (i.e., that are transmitted through the gate 100 to the destination 110, which in this case is a detector) plotted versus the injection time (the time that the gate is ON). Curve 205b is a similar plot representing the total number of ions having m/z equal to 1000 that are detected. Curve 208 in FIG. 2 is the ratio, R1000/100, of the calculated values of the detected population of heavier-mass to lighter-mass ions, also plotted versus injection time. (Note that the leftmost vertical axis represents the number of ions that are detected, whereas the rightmost axis represents the ratio.) The time t=0, at the leftmost side of the plot, is the time that the gate is switched to the ON state. Any deviation of the ratio R1000/100 from unity (denoted by a dashed horizontal line in FIG. 2) indicates a bias in the population of ions transmitted through the conventional ion gate 100. Although the ratio R1000/100 approaches unity at long gate times, FIG. 2 shows that there is a significant under-representation of the heavier ions at gate times that are on the order of or less than the flight times of ions through the gate. This transmission bias in favor of lighter ions at short injection time periods has not been a significant restriction in the past, but as technological advances cause the ion sources to become “brighter”, that is, a source of a greater ionic flux, the corresponding injection times decrease.
One way of implementing a brighter source in a mass spectrometer using electrospray ionization has been described in U.S. patent application Ser. No. 11/764,100 filed on Jun. 15 2007 and incorporated herein by reference in its entirety. In the aforementioned U.S. patent application Ser. No. 11/764,100, there is disclosed an improved means of ion transfer between the capillary and the skimmer through the provision, between the capillary and the skimmer, of a focusing device comprising a stacked ring radio-frequency (RF) ion guide with constant internal diameter. To assist in focusing ions at the exit of the stacked ring RF ion guide, either the spacing between rings is varied across the stack or the RF level is varied across the stack. Ions are moved towards the exit by means of either gas flow or an axial DC field. To prevent large clusters from flying through the focusing device, the stack can be bent such that there is no line of sight between the entrance and the exit. The focusing device may be constructed on a printed circuit board (constructed of either fiberglass or ceramic), because holes to support the electrodes can be drilled in the board at arbitrary positions to provide the variable ring spacing.
With improvements in source brightness such as discussed above, gating times need to be shortened so as to provide no more than an optimum quantity of ions to an ion trap device. One possibility for shortening the minimum gate period is to use a gate of shorter physical length. However, shorter length gates suffer the disadvantage of requiring greater voltage delivery to the electrodes in order to guarantee deflection of ions during the off period. Equivalently, ions could be moved across the gate faster using higher translational energies, but again this creates the disadvantageous situation in which larger voltages are required for acceleration as well as for complete deflection. Alternatively, the minimum time can be somewhat compensated for by adding the known or predictable flight time across the gate to the requested injection time. For example, if it is known that an ion takes 5 microseconds to cross the gate at a specified energy, then to provide 10 microseconds of ions, the gate must be held open for a total of 15 microseconds. Unfortunately, as noted above, the flight time across the gate is m/z dependent, and thus this simple compensation scheme only works for a single mass. Although such a compensation scheme would be suitable for mass spectrometry apparatuses or modes of operation in which only a single m/z is of concern during a particular gate period, such as selected ion monitoring (SIM) mass spectrometry or tandem mass spectrometry (sometimes referred to as MS/MS or MSn), these are situations where short injection times are less likely because the ions of only a single or restricted range of m/z comprise only a small fraction of the ion flux or of the ion population maintained in a storage device in front of the mass analyzer. More often, short injection times are required during full scans where all of the ions are trapped.
From the foregoing discussions, it may be observed that there is a need in the art for improved ion gate apparatuses and methods that reduce the bias, relative to conventional ion gates, in the population of ionic masses transmitted therethrough.